ATVOD said that the 13 adult services found to be in breach of the rules had mostly failed to place proper controls around their hardcore material.

Two of those have been closed down, while seven have since acted to bring their services within ATVOD's compliance requirements.

However, four services have taken no action and so have now been handed to Ofcom, ATVOD's "back-stop regulator", to "consider whether to impose a financial penalty or restrict or suspend the service".

"We have made good progress in ensuring that UK operators of regulated VOD services comply with rules designed to protect children from harmful content, but we are not complacent and will continue to monitor relevant services and act as required," said ATVOD chief executive Pete Johnson.

"Our recent enforcement activity has sent a clear message that UK providers of hardcore pornography on-demand must take effective steps to ensure that such material is not accessible to under-18s.

"Asking visitors to a website to click an 'I am 18' button or enter a date of birth or use a debit card is not sufficient - if they are going to offer explicit sex material they must know that their customers are 18, just as they would in the 'offline' world."

Alongside designating ATVOD until 2020, Ofcom has given its co-regulator more operating freedom, such as no longer needing to seek prior approval from Ofcom before publishing new guidelines.

ATVOD has seen the number of VOD services under its remit rise from 154 in its first year of 2010/11 to 184 in 2011/12. This includes all the major players such as Virgin Media and Sky, but also niche services and online community groups.

ATVOD chair Ruth Evans said: "ATVOD has developed and matured as a regulator over its first two years and we warmly welcome Ofcom's decision to reflect this not just by confirming that the Designation will run until at least 2020 but also by giving ATVOD greater autonomy and independence."

Alongside monitoring and taking action against adult services in the UK, ATVOD had been keeping an eye on how EU member states regulate hardcore porn. However, it does not have the power to regulate any material that comes from outside of Britain.

Under current EU law safeguarding the common market, adult channels are allowed to broadcast under the laws of their host country, meaning the Dutch networks escape regulation by UK watchdog Ofcom.

But MPs and campaigners have expressed concern over the content of Babestation and Smile TV, which are free to air on Freeview between 10pm and 6am, often showing images of scantily clad women rubbing each other's breasts and simulating orgasms.